1. Field of the Invention
This invention relates to the field of gas chromatography (GC), and more particularly, is directed to an apparatus and method for fabricating electrically insulated gas chromatograph components for use in gas chromatograph instruments.
The subject invention is further directed to the integration of a plurality of electrically insulated gas chromatograph component members into a gas chromatograph assembly. Of particular importance is that the gas chromatograph assemblies fabricated in accordance with the subject invention find applications in miniaturized, low thermal mass gas chromatography instruments that operate at very high temperatures.
2. Prior Art
In the known prior art systems, integration of miniature heating elements with GC columns has proven to be the most successful approach for achieving miniaturized, low thermal mass, temperature-programmable GC instruments. This approach includes: resistively-heated metal-coated columns; resistively-heated metal-walled columns; small heater plates; internal heater wire; external heater wire; resistively-heated tubing jackets; and, a resistively-heated element with a column in a sheath.
For a resistive-heating element, which runs the length of the column, including the use of the column wall as a heater, special care must be taken to prevent the occurrence of electrical shorts between the resistive-heating element or component member and other conductive components in the GC assembly along the full length of the heating element. Due to the very long lengths of such component member, for example, on the order of fifteen to thirty meters or more, electrical shorts are a constant problem when the component members are packaged and coiled together as an assembly for deployment in the GC instrument. Typically, in the prior art, wire component members are provided with an enamel or other polymeric electrical insulation layer coated or otherwise formed thereover to prevent such electrical shorting. For instance, a coating of high temperature polymer such as polyimide has been provided in some prior art systems.
The aforementioned problem of electrical short formation in the GC column assemblies is exacerbated by the inclusion therein of additional metallic elements, that are coupled to the assembly, for sensing temperature in the assembly for feedback temperature control.
Although it is possible to use the heating element itself as a temperature sensing element, such has proven impractical since the heating element must sense very small resistance changes, for example, on the order of four hundred parts per million per degree centigrade (.degree. C.), while at the same time introducing a sufficiently small resistance for the element to function as a heater. Thus, it is standard practice to provide an additional temperature sensor in the assembly which can take the form of a high resistance wire such as platinum, which provides a reproducible substantially linear resistance change with temperature. The co-location of this additional sensing wire or resistance temperature device (RTD) with the heating element or wire provides an adequate solution in one respect since the RTD wire integrates and averages the temperature along the length of the column, however, close placement of the RTD wire with the heating element or wire frequently results in the further occurrence of electrical short formation between these elements and a corresponding failure of the temperature control process.
The problem of electrical short formation is especially acute at the upper end of the standard temperature programming ranges at which capillary GC analytic columns now operate. Typical temperature ranges extending between 250-300.degree. C. are necessary for effective GC analysis, and further, new column materials are offering even higher temperature operation, in excess of 400.degree. C., for extended performance. Most wire insulations, which soften between 100.degree. C. and 200.degree. C., provide inadequate protection from electrical short formation. Further, only the "high temperature" enamel insulations are stable to 250.degree. C., however, even these insulations fail to offer extended life above 250.degree. C., and thus fail to meet temperature requirements in the GC assemblies.
The breakdown of these various insulations at the higher operating temperatures of the capillary column is intensified by the fact that the local temperatures required at the heating element or wire surface typically significantly exceed the required average GC component operating temperatures, since significant thermal conduction and convective losses occur through heating element or wire contact with other components in the GC assembly and the air. Stated otherwise, the heating wire typically runs at a much higher temperature than the average required operating temperature of the GC assembly. This is especially true during the step of temperature programming of the assembly when the heating wire is dissipating the power required to raise the average temperature of the column and other components to a predetermined target temperature. Since the heater wire runs "hotter" than the surrounding components, in order to preserve the integrity of the wire insulation, the maximum average temperature for programming of prior art GC assemblies must be set at a significantly lower level than the temperature at which the integrity of the wire insulation begins to fail or break down. Fast temperature programming of the GC assembly further worsens this problem as insulations on heater wires are easily and quickly destroyed in efforts to rapidly heat other GC components. For example, electrical shorting between enamel insulated heater and RTD wires has been a known problem with prior art tubular jacketed GC assemblies since these assemblies were introduced. With typical RTD control circuits, electrical shorts between the heater and sensor wires may lead to runaway heating conditions.
In addition to the inclusion of both metal heater wires and RTD sensor wires into the GC assembly, another current trend is to further include capillary GC columns made of a metal composition. These metal or steel capillary columns exhibit a robustness exceeding that inherent in the fused silica capillary GC columns, thus, such robust columns are of special interest for use in ruggedized instruments such as field portable GC instruments.
Due to the aforementioned problems associated with electrical shorting between heater wires and RTD wires, it has not previously been practical or indeed advisable to introduce steel capillary GC columns into wire-heated GC assemblies unless the maximum operating temperatures and temperature programming rates are kept sufficiently low so as to protect the integrity of the electrical insulation layers. Even in such limited and conservative operational circumstances, the likelihood that electrical shorts will form is still large since typical polyimide wire insulation coating is only a few thousandths of an inch thick, even for supposed "heavy" coating "builds." This structural limitation provides insufficient insulation integrity under numerous common situations which leads to frequent electrical shorting. For instance, the insulation films are so thin that manufacturing defects affecting the thickness of the insulation, stress experienced in the insulation during handling of the GC instrument components, bending of the insulation, brittleness of the coating due to age and repetitive cyclic heating, all contribute to insulation layer failures and consequent operational shorting of the metal GC columns with the other metal components. Thus, the commercially available wire insulations have not been found to meet the operational requirements of the various GC assemblies and their applications. Further intensifying these problems is the fact that due to the much greater thermal mass of steel capillary GC columns, as compared to the quartz or silica GC columns, more power is required to perform temperature programming which, as discussed previously, generates higher surface temperatures for the variety of heating elements included in the assembly.
In practice, the failure of the polyimide coated heater wire is demonstrable. Such failure occurs even when the polyimide coating is specified as a "quad" build, that is, a thick coating, rated to survive 1,000 hours at 300.degree. C. The coating quickly fails due to the high local temperatures required at the wire surface to meet overall 300.degree. C. GC temperature program requirements, as described above.